The present invention relates to a device for optically measuring the gloss of a surface, and more particularly, to a gloss gauge which is calibrated with reference to the reflectance of a surface having of a known gloss.
One of the parameters used in determining the quality of a particular surface is the surface luster or the gloss of the surface. For example, in paper production, various grades of paper having different surface gloss are produced to suit various applications. During paper production, it is desirable to periodically or continuously measure the gloss of the surface of the paper to ensure that the paper surface has the desired gloss.
Typically, the surface gloss of paper is measured using a gloss gauge during the last step of paper production before the finished paper, which is manufactured in a continuous sheet, is packaged in the form of rolls. The rolls of paper are then shipped to paper products manufacturers who process the paper sheet in accordance with the intended use.
Certain devices for determining the gloss of paper surfaces comprise an optical system which measures the intensity of a beam of light reflected from the paper surface. Typically, the gloss of the paper surface is determined by comparing its reflectance to the reflectance of a known gloss standard, such as, for example, a glass tile having a polished surface with a known gloss. Specifically, in measuring the reflectance of the paper surface, light is projected onto the surface, and a sensor which is responsive to the intensity of light is positioned to measure the intensity of the light reflected from the paper surface. The gloss gauge measures the reflectance of the tile surface in the same manner by substituting the tile surface for the paper surface. The reflectance of the paper surface is referenced to the reflectance of the tile surface, thereby providing a measure of the gloss of the paper surface. In practice, the reflectance measurement of the tile surface is periodically performed, off-sheet and between scans, as the gloss gauge scans back and forth across the paper surface. The gloss gauge is calibrated during each such measurement with the known reflectance of the tile surface.
Certain gloss gauges require relatively complex mechanisms for maneuvering the standardization tile into position for a reflectance measurement and then moving the tile out of the way of the light beam for reflectance measurements of the paper surface. Moreover, because of their complexity, it is difficult for such mechanisms to maintain a proper constant angular alignment of the tile with respect to the optical system of the gloss gauge. Such alignment is critical to proper reflectance measurements since reflectance is very sensitive to changes in angular alignment of the reflecting surface.
Also, the tile surface used in certain gloss gauges is subject to dust build-up. Dust particles, generated by the paper mill during the paper production process, may cover the tile surface. The dust particles alter the reflectance of the tile surface. As a result, a gloss gauge having a dust-contaminated tile surface will not provide a true comparison of the reflectance of the tile surface to the paper surface. Frequent cleaning of the tile surface has therefore been required to maintain an acceptable gloss standard.